Disk drive spindle motor having split windings for each phase

ABSTRACT

A reduced size brushless DC motor preferably for use as a disk drive spindle motor includes a rotor having two sets of permanent magnets, and a stator having separate portions corresponding to the two sets. An upper set of permanent magnets and the corresponding portion of the stator are located within the upper portion of the hub, which fits within the holes of the disks of a disk drive. A lower set of permanent magnets and corresponding portion of stator are located in the flange of the motor hub which supports the disks from below. In one embodiment, the stator core has an L-shaped cross-sectional area in the plane of the motor axis, one leg of the L driving the upper set of magnets in the rotor and the other leg driving the flange magnets. In a second embodiment, the stator core includes two separate pieces, one corresponding to each set of permanent magnets. In either embodiment, the separate coils or coil portions may be electrically connected in series or in parallel, or may be connected to switches enabling them to be driven in series or parallel selectively. Furthermore, the phases of the two sets of permanent magnets may be staggered to reduce the effects of transition from one magnetic pole to the next. Where the stator core includes two separate pieces, it is also possible to stagger the stator phases as well, reducing torque pulsations and wire interference.

This is a continuation of co-pending application Ser. No. 08/286,760filed on Aug. 5, 1994.

FIELD OF THE INVENTION

The present invention relates to electric motors, and in particular tobrushless DC electric motors of the type commonly used in disk drivestorage devices.

BACKGROUND OF THE INVENTION

The extensive data storage needs of modern computer systems requirelarge capacity mass data storage devices. A common storage device is therotating magnetic disk drive.

A disk drive typically contains one or more smooth, flat disks which arerigidly attached to a common spindle. The disks are stacked on thespindle parallel to each other and spaced apart so that they do nottouch. The disks and spindle are rotated in unison at a constant speedby a spindle motor.

Each disk is formed of a solid disk-shaped base or substrate, having ahole in the middle for the spindle. The substrate is commonly aluminum,although glass, ceramic, plastic or other materials are possible. Thesubstrate is coated with a thin layer of magnetizable material, and mayadditionally be coated with a protective layer.

Data is recorded on the surfaces of the disks in the magnetizable layer.To do this, minute magnetized patterns representing the data are formedin the magnetizable layer. The data patterns are usually arranged incircular concentric tracks. Each track is further divided into a numberof sectors. Each sector thus forms an arc, all the sectors of a trackcompleting a circle.

A moveable actuator positions a transducer head adjacent the data on thesurface to read or write data. The actuator may be likened to the tonearm of a phonograph player, and the head to the playing needle.

There is one transducer head for each disk surface containing data. Thetransducer head is an aerodynamically shaped block of material (usuallyceramic) on which is mounted a magnetic read/write transducer. Theblock, or slider, flies above the surface of the disk at an extremelysmall distance as the disk rotates. The close proximity to the disksurface is critical in enabling the transducer to read from or write tothe data patterns in the magnetizable layer. Several differenttransducer designs are used, and in some cases the read transducer isseparate from the write transducer.

The actuator usually pivots about an axis to position the head. Ittypically includes a solid block near the axis having comb-like armsextending toward the disk a set of thin suspensions attached to thearms, and an electro-magnetic motor on the opposite side of the axis.The transducer heads are attached to the suspensions, one head for eachsuspension. The actuator motor rotates the actuator to position the headover a desired data track. Once the head is positioned over the track,the constant rotation of the disk will eventually bring the desiredsector adjacent the head, and the data can then be read or written.

As computer systems have become more powerful, faster, and morereliable, there has been a corresponding increase in demand for improvedstorage devices. These desired improvements take several forms. It isdesirable to increase data capacity, to increase the speed at which thedrives operate, to reduce the electrical power consumed by the drives,and to increase the resilience of the drives in the presence ofmechanical shock and other disturbances.

In particular, there is a demand to reduce the physical size of diskdrives. To some degree, reduction in size may serve to further some ofthe above goals. But at the same time, reduced size of disk drives isdesirable in and of itself. Reduced size makes it practical to includemagnetic disk drives in a range of portable applications, such as laptopcomputers, mobile pagers, and "smart cards".

An example of size reduction is the application of the PCMCIA Type IIstandard to disk drives. This standard was originally intended forsemiconductor plug-in devices. With improvements to miniaturizationtechnology, it will be possible to construct disk drives conforming tothe PCMCIA Type II standard.

In order to shrink the size of disk drives, every component must bereduced in size as much as possible. Size reduction of a component cannot always be accomplished by merely scaling down. This is true inparticular of the spindle motor which rotates the disk.

Disk drive spindle motors are typically brushless direct current (DC)motors. These motors have a stationary set of electromagnets (thestator), and a set of permanent magnets attached to the rotating part ofthe motor (the rotor). The electromagnets of the stator are arranged ina circle surrounding the motor shaft. Each electromagnet includes a core(usually made of iron) surrounded by a coil (or winding) of electricwires. A motor controller, which is a set of switches and logic circuitson one or more circuit chips, sends pulses of electric current throughthe different coils to energize the electromagnets. The electromagnetsare energized in a sequential pattern travelling around the shaft,inducing the permanent magnets in the rotor to follow, thus imparting arotational force to the rotor.

Conventional disk drive spindle motors have generally employed one oftwo physical designs. In the first design, sometimes called a "pancake"motor, the stator electromagnets and rotor magnets are positioned underthe disk stack. In order to reduce the overall height of the disk drive,the motor is made as flat and elongated as possible, giving it the name"pancake". However, even with a flattened motor, the design inevitablyadds something to the overall height of the drive. In the second designthe stator electromagnets and rotor magnets are contained within theholes of the disks on the disk stack. This design is referred to as an"in-hub" motor. An in-hub motor permits the overall height of the diskdrive to be reduced, but it requires that the holes in the disks besufficiently large to accommodate the motor components. The larger theholes, the less area that is available on the surface of the disks forrecording data.

When attempting to shrink the size of conventional disk drive designs,the size of the spindle motor becomes a severely limiting factor. Aconventional pancake motor is undesirable due to the need to reduce diskdrive height, particularly in the case of the PCMCIA Type II formfactor. An in-hub motor would appear to be the preferred design.However, the rotational force (torque) that the motor is capable ofgenerating is related to the distance from the permanent magnet to theaxis of rotation. As the in-hub motor components are squeezed into thesmall space available in the hole of a smaller disk, it becomesdifficult to generate the torque needed for proper operation.

It is possible to design motors for PCMCIA drives using conventionaltechniques, but such designs involve trade-offs with other goals. Torquecan be increased by increasing the amount of electric current pulsedthrough the windings, but this increases the power consumption of thedrive and requires larger electronic components for pulsing thewindings. Another solution is to enlarge the holes in the disks toincrease the distance from the rotor magnets to the axis, therebyincreasing the torque, but this reduces the area of the disk availablefor storing data. Alternatively, the motor can be operated at a slowerspeed, thereby reducing the amount of torque required, but this causesthe disk drive to access data more slowly. It is desirable to develop amore compact disk motor design which reduces the need for these designtrade-offs.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide anenhanced disk drive storage apparatus.

Another object of this invention is to provide an electric motor havinga more compact size.

Another object of this invention is to provide an enhanced spindle motorfor a disk drive storage device.

Another object of this invention is to provide a reduced size spindlemotor for a disk drive storage device.

A brushless DC motor preferably for use as a spindle motor of a diskdrive storage device includes a rotor having two sets of permanentmagnets, and a stator having separate portions which drive thecorresponding sets of permanent magnets. An upper set of permanentmagnets and the corresponding portion of the stator are located withinthe main body or upper portion of the hub, which fits within the holesof the disk or disks of a disk drive. A lower set of permanent magnetsand corresponding portion of stator are located in the flange of themotor hub which supports the disks from below. The lower set of magnetsis thus positioned at a greater radial distance from the axis than theupper set, thereby increasing torque over what would be possible frommagnets confined to the disk hole alone. We call this motor design the"double-decker" configuration.

In one embodiment, the stator core has an L-shaped cross-sectional areain the plane of the motor axis, one leg of the L extending into theupper part of the hub and the other extending into the flange under thedisks. A portion of the coil surrounds the upper leg in the longitudinaldirection, while another portion of the coil surrounds the lower legperpendicular to its longitudinal direction. The first leg of the coredrives the upper set of permanent magnets mounted within the holes ofthe disks, while the second leg drives a set of permanent magnetsmounted in the flange.

In a second embodiment, the stator core includes two separate pieces,one corresponding to each set of permanent magnets. The separate corepieces are wound with separate coils.

In either embodiment, the separate coils or coil portions may beelectrically connected in series or in parallel, or may be connected toswitches enabling them to be driven in series or parallel selectively.Furthermore, the phases of the two sets of permanent magnets may bestaggered to reduce the effects of transition from one magnetic pole tothe next. Where the stator core includes two separate pieces, it is alsopossible to stagger the stator phases as well, reducing torquepulsations and wire interference.

Because the hub normally requires a flange at the bottom to support thedisks, the location of a part of the motor within the flange can beaccomplished with very little wasted space. Furthermore, the permanentmagnet located within the flange is located at a considerably greaterdistance from the axis than is possible for the magnet located withinthe upper part of the hub, thus achieving significant increase in motortorque.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a magnetic disk drive storage unit according to thepreferred embodiment;

FIG. 2 is a sectional view of the hub assembly including spindle motoraccording to the preferred embodiment;

FIG. 3 is a sectional view of the hub assembly including spindle motoraccording to an alternative embodiment;

FIG. 4 is a sectional view of the rotor magnet assembly before insertioninto the rotor, according to the preferred embodiment;

FIG. 5 shows the rotor magnet assembly as viewed in a planeperpendicular to the axis of disk rotation according to the preferredembodiment;

FIG. 6 shows the major components of electronic drive circuitry for thespindle motor of the preferred embodiment;

FIG. 7 shows in greater detail the configuration of stator windingsaccording to the preferred embodiment;

FIG. 8 shows in greater detail the configuration of stator windings inan alternative embodiment;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a magnetic disk drive storage unit 100 in accordance withthe preferred embodiment. Disk unit 100 comprises rotatable disk 101,which is rigidly attached to hub assembly or spindle 103, which ismounted on disk drive base or housing 104. Spindle 103 and disk 101 aredriven by a drive motor at a constant rotational velocity. The drivemotor is contained within hub assembly 103. Actuator assembly 105 issituated to one side of disk 101. Actuator 105 rotates through an arcabout shaft 106 parallel to the axis of the spindle, driven byelectro-magnetic motor 107, to position the transducer heads. A cover(not shown) mates with base 104 to enclose and protect the disk andactuator assemblies. Electronic modules for controlling the operation ofthe drive and communicating with another device, such as a hostcomputer, are mounted on a circuit card 112 within the head/diskenclosure formed by base 104 and the cover. In this embodiment, circuitcard 112 is mounted within the enclosure and shaped to take up unusedspace around the disk in order to conserve space, as would be used for aPCMCIA Type II form factor. However, the card 112 could also be mountedoutside the head/disk enclosure, or the base itself could be made as acircuit card for mounting electronic modules directly to it. A pluralityof head/suspension assemblies 108 are rigidly attached to the prongs ofactuator 105. An aerodynamic read/write transducer head 109 is locatedat the end of each head/suspension assembly 108 adjacent the disksurface.

While disk drive 100 is shown with a single disk such as would be usedfor a PCMCIA Type II form factor, it should be understood that thepresent invention could utilize a drive having multiple disks mounted onthe spindle.

FIG. 2 shows in greater detail hub assembly 103 and related hardware,including components of the spindle motor, in accordance with thepreferred embodiment. FIG. 2 is a half sectional view, taken in theplane of the axis of rotation of the disks. Disk shaft 202 is rigidlyattached to motor base 203, which is in turn rigidly attached to base104 of the disk drive. Motor base 203 and disk drive base 104 mayalternatively be integrally formed as a single piece to conserveadditional space. Shaft 202 is preferably steel. Disk axis 201 runsthrough the center of shaft 202. While hub assembly 103 on only one sideof the axis is depicted in FIG. 2, it should be understood that hubassembly 103 is symmetrical about the axis. Rotor housing 205 is mountedon bearing assembly 204 for rotation about axis 201. Bearing assemblypreferably comprises two sets of ball bearings in sealed beating races,at opposite ends of shaft 202. However, bearing assembly couldalternatively be a fluid (hydrodynamic) or other type of bearing.

Rotor housing 205 has a roughly cylindrical main body (upper or hubportion), and a cylindrical flange portion 205A extending outward fromthe main body thereof, near the bottom. Flange portion 205A supportsdisk 101 from below. Rotor housing is preferably steel to provide amagnetically permeable back iron for the permanent magnets. However,other materials such as plastic may also be used, with or without aseparate back iron member. A clamp mechanism (not shown) applies anaxial force downward on the disk, pressing it against the flange andholding it in place. Where multiple disks are used, spacers areinterposed between each disk and the clamping mechanism presses theentire disk stack against the flange. Various clamping mechanisms andspacers are known in the art.

Housing 205 is hollow for placement of motor components. Upper permanentmagnet 211 is fastened to the inside of rotor housing 205 within thehole of disk 101. Lower permanent magnet 212 is fastened to the insideof flange portion 205A of housing 205. Connecting ring 213 made of anon-magnetic material connects lower magnet 212 to upper magnet 211.

Motor core 221 preferably comprises a series of laminations of amagnetically permeable material, built up to have an L-shapedcross-section as shown. The laminations are preferably made from siliconsteel, although any appropriate magnetically permeable material may beused. Upper leg portion 221A of core 221 extends into the upper part ofhousing 205, within the hole of disk 101. Lower leg portion 221B of core221 extends outward from the axis into flange portion 205A of housing205.

Motor coils or windings 224,225 surround core 221 to form the statorelectromagnet. The wires of windings 224,225 are wound substantiallyparallel to disk axis 201. Winding 224 encircles upper leg core 221A inthe longitudinal (axial) direction. I.e., wires of winding 224 generallyrun from the top of upper leg core 221A to the bottom of core 221A andback again. Winding 225, which is substantially parallel to winding 224,encircles lower leg core 221B in the short direction, from top to bottomand back.

FIG. 3 shows an alternative embodiment of hub assembly 103 and relatedhardware. Specifically, in the alternative embodiment rotor housing 205and magnets 211,212 are the same as in the preferred embodiment.However, the stator is constructed differently. The stator of thealternative embodiment comprises two separate core pieces 321,322, whichdrive magnets 211,212 respectively. Windings 324,325 encircle therespective core pieces. As is true of the preferred embodiment, windings324,325 of the alternative embodiment may be electrically connected inseries or in parallel, or may be separately switched.

In either embodiment depicted in FIGS. 2 or 3, permanent magnets 211,212are preferably assembled to non-magnetic connecting ring 213 beforeinsertion into rotor housing 205. FIG. 4 is a sectional view of thepermanent magnet assembly before insertion. Connecting ring 213 ispreferably nylon, although other materials could be used. Magnets211,212 are preferably attached to non-magnetic connecting ring 213 witha suitable adhesive. Preferably, magnetic material for the magnets isfirst assembled to the non-magnetic ring, a permanent magnetic field isthen imposed on the magnets, and they are then inserted into the rotor.This method of assembly assures that a precise circumferential alignment(including optional offset) can be obtained between the upper and lowersets of magnets.

The double-decker motor configuration of the present invention providesan added benefits through an additional degree of freedom in the motordesign. Because the motor spins in only one direction, the two sets ofstator and rotor assemblies can be purposely staggered in phase toimprove the torque ripple characteristics and motor acoustics. In thepreferred embodiment, this can be accomplished by circumferentiallyoffsetting the two sets of permanent magnets 211,212.

FIG. 5 shows the rotor magnet assembly as viewed in a planeperpendicular to the axis of disk rotation. Each set of permanentmagnets 211,212 preferably comprises 12 individual magnet poles ofalternating polar orientation, which collectively surround the diskaxis. The upper set of magnets 211 surrounds the axis at a shorterradius than the lower set 212. Preferably, the two sets are slightlyoffset circumferentially, as shown. Where 12 magnet poles per set areused as in the preferred embodiment, and there are nine stator windingsin each set, it is preferred that the offset between the two sets ofmagnets be approximately five degrees. However, the amount of preferredoffset will vary with the number of phases in the stator windings, thenumber of magnets in each set, and other design considerations. It wouldalso be possible to construct the motor without any offset.

FIG. 6 shows the major components of electronic drive circuitry for thespindle motor of the preferred embodiment. Preferably, the statorwindings are connected in a 3-phase wye configuration 601 having acentral tap, although a delta configuration could also be used. Thethree phases of the stator windings are driven by switchable 3-phasecurrent driver circuits 602. Driver circuits 602 drive windings selectedby commutation control circuit 603 at the current specified by a currentsignal. Commutation control circuit 603 receives feedback from thedriver phases and central tap to sequentially switch the phases ofcurrent driver circuits 602 as the motor rotates. Commutation control603 also provides a velocity feedback signal to velocity control circuit604. Velocity control circuit 604 also receives a motor current feedbacksignal from current driver circuits 602. Velocity control circuitcompares actual velocity to a desired reference velocity input frome.g., a disk drive microprocessor controller. Velocity control circuit604 adjusts the rotational velocity of the motor to conform to thereference velocity by varying an input current signal to the currentdriver circuits 602. Preferably, drive circuit components 602-604 arelocated on common circuit card 112 with other disk drive electroniccomponents. While drive circuit components 602-604 are shown as separateblocks for illustrative purposes, it should be understood that theycould be contained on a common circuit chip, and may share a chip withother disk drive circuitry.

FIG. 7 shows in greater detail the configuration of stator windings 601.Preferably, stator windings 601 are arranged as shown with three polesper phase, or a total of nine poles 701-709. All windings of a singlephase are in series. In the preferred embodiment, in which core 221 hasan L-shaped cross-sectional area as shown in FIG. 2, windings 224,225are in reality portions of a single pole winding 701, the portions224,225 being wound in series. The winding configuration of FIG. 7 mayalso be used in the alternate embodiment of FIG. 3, in which casewindings 324,325 in each pole are connected in series.

FIG. 8 shows an alternative winding configuration, in which all poles ofthe lower windings 801-809 are connected in series before the poles ofthe lower windings 811-819. The configuration of FIG. 8 is particularlyapplicable to the alternate embodiment of core pieces shown in FIG. 3.Because core pieces are separated in this embodiment, separation ofupper and lower windings is more easily accomplished.

While windings shown in the preferred embodiment are connected inseries, it should be understood that windings could alternatively beconnected in parallel, either as individual poles or groups of upper andlower poles. For example, it would be alternatively possible to connectwinding poles 701,704 and 707 in parallel with the Phase A drivecircuit. As another alternative, it would be possible to connect groupof winding poles 801,804 and 807 in series and group of winding poles811,814 and 817 in series, and connect the two groups in parallel withthe Phase A drive circuit. It would additionally be possible for thedrive circuitry to selectively switch between serial and parallelconnections to obtain maximum torque at motor start-up and maximumefficiency at running speed, as described in commonly assigned copendingpatent application Ser. No. 08/109,752 to Apuzzo, filed Aug. 19, 1993,herein incorporated by reference. It would additionally be possible toselectively power the two sets of stator windings, for example, to powerboth sets during start-up or conditions of peak load, but to power onlyone set at other times.

It would alternatively be possible to stagger the sets of statorwindings, where separate windings are used as depicted in FIG. 3. Insuch an alternative embodiment, the two sets of separate windings wouldbe connected as shown in FIG. 8, but the two sets of windings would becircumferentially offset from one another in a manner similar to theoffset of permanent magnets shown in FIG. 5.

In the preferred embodiment, a configuration of 12 permanent magnetpoles in the rotor and 9 stator poles (3 phases having three poles each)is used. However, it will be understood by those skilled in the art thatthe number of permanent magnet poles, stator poles and stator phases mayvary. A larger number of poles will generally reduce noise andvibration, but becomes more difficult to construct, particularly wherethe parts must be very small as with a PCMCIA Type II disk drive. The12/9 configuration is chosen for the preferred embodiment as appropriatefor a small form factor disk drive, but other configurations may be moresuited to different applications.

In the description above, the narrower main body of the rotor housingand related components have sometimes been referred to as the "upper"portion or set, while the wider flange and related components have beencalled the "lower" portion or set. "Upper" and "lower" are used only forease of reference and are consistent with the normal orientation used inthe art. However, the use of these terms is not meant to imply that thepresent invention requires the flange to be located below the main bodyportion of the rotor. The motor and disk drive of the present inventioncould just as easily be constructed with the flange located above themain body, or with the axis of rotation oriented horizontally.

In the preferred embodiment, the electric motor of the present inventionis used as a spindle motor of a disk drive storage device. As explainedabove, the demand for miniaturized devices of this type provides animpetus for extremely compact motors. However, it would be possible touse such a motor in other applications requiring a very compact motordesign, such as special purpose pumps, fans, etc. Furthermore, while inthe preferred embodiment the storage devices are rotating magnetic diskdrive storage devices, it should be understood that the presentinvention may be applicable to optically read disk devices such asCD-ROM or disk storage using other recording technologies.

Although a specific embodiment of the invention has been disclosed alongwith certain alternatives, it will be recognized by those skilled in theart that additional variations in form and detail may be made within thescope of the following claims.

What is claimed is:
 1. An electric motor, comprising:a base; a rotorhousing rotatably mounted to said base for rotation about an axis, saidrotor housing having a substantially cylindrical first hub portionhaving a first diameter, and a substantially cylindrical second hubportion having a second diameter, said second diameter being greaterthan said first diameter, said first and second hub portions beingcylindrical about said axis; a first set of permanent magnets mountedwithin said first hub portion of said rotor housing and surrounding saidaxis at a first radial distance; a second set of permanent magnetsmounted within said second hub portion of said rotor housing andsurrounding said axis at a second radial distance, said second radialdistance being greater than said first radial distance; anelectromagnetic stator assembly rigidly mounted to said base andsurrounding said axis, said stator including a first stator portion fordriving said first set of permanent magnets, and a second stator portionfor driving said second set of permanent magnets.
 2. The electric motorof claim 1, wherein said first set of permanent magnets iscircumferentially offset from said second set of permanent magnets. 3.The electric motor of claim 1, wherein said electromagnetic statorassembly comprises a plurality of cores surrounding said axis, and atleast one winding surrounding each respective core, each core having asubstantially L-shaped cross-section in a plane containing said axis,forming two legs of said core, a first leg of said core being adjacentsaid first set of permanent magnets, and a second leg of said core beingadjacent said second set of permanent magnets, said first stator portioncomprising the first leg of each respective core, and said second statorportion comprising the second leg of each respective core.
 4. Theelectric motor of claim 1, wherein said electromagnetic stator assemblycomprises a first set of cores and a second set of cores, each set ofcores comprising a plurality of cores surrounding said axis, said statorassembly further comprising at least one winding surrounding eachrespective core, said first set of cores being positioned adjacent saidfirst set of permanent magnets and said second set of cores beingpositioned adjacent said second set of permanent magnets, said firststator portion comprising said first set of cores, and said secondstator portion comprising said second set of cores.
 5. The electricmotor of claim 4, wherein said first set of cores is circumferentiallyoffset from said second set of cores.
 6. The electric motor of claim 4,wherein said first set of permanent magnets is circumferentially offsetfrom said second set of permanent magnets.
 7. The electric motor ofclaim 4, wherein said first and second set of cores are selectivelypowered.
 8. The electric motor of claim 1, further comprising anon-magnetic annular member mounted within said rotor housing and towhich said first and second sets of permanent magnets are attached, saidfirst set of permanent magnet being attached to a first surface of saidannular member near its inner edge, and said second set of permanentmagnets being attached to a second surface of said annular member nearits outer edge, said annular member fixing the relative circumferentialpositions of said first and second sets of permanent magnets.
 9. Theelectric motor of claim 1, wherein said rotor housing further comprisesa supporting structure for providing mechanical support to a rotatingmember attached to said rotor housing.